Positive electrode active material for non-aqueous electrolyte secondary cell and cell using the same

ABSTRACT

There is provided a positive electrode active material for a non-aqueous electrolyte secondary battery, comprising a composite oxide represented by the general formula (1): (Li 1−x M x ) a (Co 1−y M y ) b O c  where lithium and cobalt are partly replaced with element M in the crystal structure of LiCoO c , wherein element M is at least one selected from the group consisting of Al, Cu, Zn, Mg, Ca, Ba and Sr, and 0.02≦x+y≦0.15, 0.90≦a/b≦1.10, and 1.8≦c≦2.2 are satisfied.

TECHNICAL FIELD

[0001] The present invention relates to a positive electrode activematerial for a non-aqueous electrolyte secondary battery and anon-aqueous electrolyte secondary battery for use in, for example,personal digital assistants, portable electronic appliances and home-usepower storage devices as well as two-wheeled motor vehicles, electricvehicles and hybrid electric vehicles, each of which employs a motor asthe power source.

BACKGROUND ART

[0002] Non-aqueous electrolyte secondary batteries, which have beenrecently used in various fields, have high electromotive force and highenergy density. As the positive electrode active material fornon-aqueous electrolyte secondary batteries, lithium cobaltate (LiCoO₂)is mainly used. This substance has a high oxidation-reduction potentialof 4 V or higher with respect to lithium.

[0003] The above positive electrode active material repeatedly expandsand contracts during charge/discharge. In such an occasion, the positiveelectrode active material undergoes distortion, structural destructionand pulverization, resulting in a problem of a decrease in the dischargecapacity during charge/discharge cycles. In order to solve this problem,attempts have been made to stabilize the crystal lattice to improve thecycle characteristic by partly replacing cobalt with a differentelement.

[0004] For example, Japanese Unexamined Patent Publication Nos. hei5-242891, hei 6-168722 and hei 11-7958 describe that partly replacingcobalt in the active material with a different element improves thecycle characteristic and safety of the battery. However, although thecycle characteristic is improved when cobalt is partly replaced with adifferent element, the battery thickness gradually increases owing torepeated charge/discharge cycles. In the case of square batteries orlaminated batteries, the problem of an increase in battery thicknessneeds to be addressed because the cases thereof have poor strength.

[0005] On the other hand, Japanese Unexamined Patent Publication No. hei7-226201 describes an effect of partly replacing lithium in the activematerial with a different element. However, also in this case, thebattery thickness gradually increases owing to repeated charge/dischargecycles.

[0006] Although the cause of an increase in battery thickness isunknown, it is presumably attributed to the weak interaction in thecrystal structure of the active material between a metal oxide layercomprising the aforementioned different element and cobalt and a layercomprising lithium. It is considered that repeated charge/dischargecycles increase distortion between these layers, resulting in anexpansion of the crystal lattice.

[0007] As described above, since conventional positive electrode activematerials undergo distortion, structural destruction and pulverizationdue to repeated charge/discharge cycles, they have a disadvantage ofincreasing the battery thickness and decreasing the discharge capacity.As a result, although the batteries have high thermal stability duringthe initial charge/discharge period, they have another disadvantage thatthe structure of the positive electrode active materials become unstableto cause an insufficient thermal stability after repeatedcharge/discharge cycles.

DISCLOSURE OF INVENTION

[0008] The present invention solves the foregoing problems and it has anobject of improving a positive electrode active material to suppress anincrease in battery thickness due to repeated charge/discharge cycles,thereby retaining the discharge capacity and thermal stability of abattery.

[0009] More specifically, the present invention relates to a positiveelectrode active material for a non-aqueous electrolyte secondarybattery, comprising a composite oxide represented by the general formula(1):

(Li_(1−x)M_(x))_(a)(Co_(1−y)M_(y))_(b)O_(c)  (1)

[0010] where lithium and cobalt are partly replaced with element M inthe crystal structure of LiCoO_(c),

[0011] wherein element M is at least one selected from the groupconsisting of Al, Cu, Zn, Mg, Ca, Ba and Sr, and 0.02≦x+y≦0.15,0.90≦a/b≦1.10, and 1.8≦c≦2.2 are satisfied.

[0012] It is preferable that the composite oxide has an α—NaFeO₂ typelayer structure belonging to space group R3-m.

[0013] It is preferable that the composite oxide has a mean particlediameter of 5 to 20 μm.

[0014] It is preferable that the composite oxide has a specific surfacearea of 0.3 to 1.2 m²/g.

[0015] The present invention also relates to a non-aqueous electrolytesecondary battery comprising: a positive electrode comprising theabove-described positive electrode active material; a negative electrodecomprising metallic lithium or a material capable of absorbing anddesorbing lithium; and a non-aqueous electrolyte.

BRIEF DESCRIPTION OF DRAWINGS

[0016]FIG. 1 is a partially cutaway oblique view of a square battery ofexamples in accordance with the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0017] The positive electrode active material for a non-aqueouselectrolyte secondary battery in accordance with the present inventioncomprises a composite oxide represented by the general formula (1):

(Li_(1−x)M_(x))_(a)(Co_(1−y)M_(y))_(b)O_(c)  (1)

[0018] where lithium and cobalt are partly replaced with element M inthe crystal structure of LiCoO_(c),

[0019] wherein element M is at least one selected from the groupconsisting of Al, Cu, Zn, Mg, Ca, Ba and Sr, and 0.02≦x+y≦0.15,0.90≦a/b≦1.10, and 1.8≦c≦2.2 are satisfied.

[0020] Partly replacing lithium and cobalt with element M strengthensthe binding force between a metal oxide layer comprising element M andcobalt and a layer comprising element M and lithium. As a result, anincrease in the distortion between the layers and the expansion of thecrystal lattice due to repeated charge/discharge cycles are suppressed.With the use of this active material, it is possible to produce abattery with high thermal stability of which increase in thickness dueto repeated charge/discharge cycles is suppressed to improve the cyclelife.

[0021] It should be noted that since the above-mentioned conventionalpositive electrode active materials are produced by replacing eitheronly one of lithium and cobalt in LiCoO₂ with a different element, theyare considered to be incapable of suppressing an increase in thedistortion between the layers constituting the crystal and the expansionof the crystal lattice.

[0022] When x+y, which indicates the amount of element M, is more than0.15 in the general formula (1), the structure of the positive electrodeactive material is further stabilized, whereas a decrease in the batterycapacity becomes greater. When x+y is less than 0.02, the structure ofthe positive electrode active material is not able to be stabilized.Further, from the viewpoint of stabilizing the active material structurewhile retaining the battery capacity, it is preferable that0.002≦x≦0.08, and 0.018≦y ≦0.148. When x is either too small or toolarge, it is impossible to sufficiently strengthen the binding forcebetween the oxide layer comprising element M and cobalt and the layercomprising element M and lithium.

[0023] Next, descriptions are made on the method of synthesizing acomposite oxide represented by the formula (1).

[0024] Firstly, a solution containing the starting materials of cobaltand element M is prepared. The molar ratio of Co and M contained in thesolution is set at 1-y:y. While the materials described below may beused as the starting materials of cobalt and element M without anylimitation, sulfate is preferably used herein. An alkaline aqueoussolution is continuously added dropwise to this solution, whilecontrolling the pH thereof, to precipitate a coprecipitate (hereinafter,referred to as “precursor”) comprising a cobalt hydroxide and ahydroxide of element M.

[0025] Next, the starting materials of lithium and element M are addedwith the precursor such that the molar ratio of Li and M is 1-x:x.Herein, the total number of moles of Co and M contained in the precursorand the total number of moles of Li and M contained in the startingmaterials are set to be substantially equal. Subsequently, the obtainedmixture is baked to produce a desired composite oxide.

[0026] The resultant composite oxide has a structure in which lithiumand cobalt are partly replaced with element M in the crystal structureof LiCoO_(c), for the following reasons.

[0027] Firstly, in the hydroxide synthesized as the precursor, cobaltand element M have a hexacoordinate structure having hydroxide groups asligand. Accordingly, these hydroxides are entirely placed in the sitesof cobalt of LiCoO₂, reflected by the hexacoordinate structure.

[0028] Secondly, since the total number of moles of Li and M containedin the starting materials to be mixed with the precursor is set to besubstantially equal to the total number of moles of cobalt and Mcontained in the precursor, the element M added later was entirelyplaced in the sites of lithium of LiCoO₂.

[0029] These facts can be confirmed by Rietveld analysis of the crystalstructure of the resultant composite oxide. It is also possible toconfirm that the resultant composite oxide has an α—NaFeO₂ type layerstructure belonging to space group R3-m, as in the case of LiCoO₂.

[0030] It is preferable that the composite oxide has a diameter of 5 to20 μm, from the viewpoint of stabilizing the electrode reaction. Whenthe mean particle diameter is less than 5 μm, the amount of gasgeneration on the positive electrode is increased to deteriorate thecycle characteristic. When the mean particle diameter is more than 20μm, the performance at high load and battery capacity are reduced.

[0031] It is preferable that the composite oxide has a specific surfacearea of 0.3 to 1.2 m²/g, from the viewpoint of stabilizing the electrodereaction. When the specific surface area is less than 0.3 m²/g, theperformance at high load and battery capacity are decreased, and when itis more than 1.2 m²/g, the amount of gas generation on the positiveelectrode is increased to deteriorate the cycle characteristic.

[0032] As the starting material of cobalt used for synthesizing thecomposite oxide, basic cobalt carbonate, cobalt hydroxide, cobaltnitrate, cobalt sulfate, cobalt oxide, cobalt fluoride or the like canbe used.

[0033] As the starting material of element M used for synthesizing thecomposite oxide, the following can be used.

[0034] When element M is Al, aluminium hydroxide, aluminium nitrate,aluminium oxide, aluminium fluoride, aluminium sulfate or the like canbe used.

[0035] When element M is Cu, copper oxide, copper sulfate, coppercarbonate, copper acetate, copper oxalate, copper chloride, coppersulfide or the like can be used.

[0036] When element M is Zn, zinc oxide, zinc acetate, zinc chloride,zinc fluoride, zinc sulfate, zinc nitrate, zinc sulfide or the like canbe used.

[0037] When element M is Mg, magnesium oxide, basic magnesium carbonate,magnesium chloride, magnesium fluoride, magnesium nitrate, magnesiumsulfate, magnesium acetate, magnesium oxalate, magnesium sulfide,magnesium hydroxide or the like can be used.

[0038] When element M is Ca, calcium oxide, calcium chloride, calciumcarbonate, calcium fluoride, calcium nitrate, calcium sulfate, calciumsulfide, calcium hydroxide or the like can be used.

[0039] When element M is Ba, barium oxide, barium chloride, bariumcarbonate, barium fluoride, barium oxalate, barium nitrate, bariumsulfate, barium sulfide or the like can be used.

[0040] When element M is Sr, strontium oxide, strontium chloride,strontium carbonate, strontium oxalate, strontium fluoride, strontiumsulfate, strontium nitrate, strontium hydroxide, strontium sulfide orthe like can be used.

[0041] As the starting material of lithium used for synthesizing thecomposite oxide, lithium carbonate, lithium hydroxide, lithium nitrate,lithium sulfate, lithium oxide or the like can be used.

[0042] Since its increase in thickness due to repeated charge/dischargecycles is suppressed, a non-aqueous electrolyte secondary batterycomprising: a positive electrode comprising the above-described positiveelectrode active material; a negative electrode; a non-aqueouselectrolyte is capable of retaining the discharge capacity and thermalstability over a long period.

[0043] As the negative electrode material, metallic lithium or amaterial capable of absorbing and desorbing lithium is employed.Examples of a material capable of absorbing and desorbing lithiuminclude: alloy materials; thermally decomposed carbons; cokes such aspitch coke, needle coke and petroleum coke; graphites; glassy carbons;baked organic polymer compounds produced by baking phenolic resin, furanresin or the like at a suitable temperature; carbon fibers; activatedcarbons; polymers such as polyacetylene, polypyrrole and polyacene;lithium-containing transition metal oxides such as Li₄Mn₅O₁₂; andlithium-containing transition metal sulfides such as TiS₂.

[0044] The non-aqueous electrolyte is prepared by dissolving a lithiumsalt in a non-aqueous solvent.

[0045] As the non-aqueous solvent, any known materials and additives canbe employed. Particularly preferred are: a mixed solvent of a cycliccarbonate such as ethylene carbonate, propylene carbonate, butylenecarbonate or vinylene carbonate and a non-cyclic carbonate such asdimethyl carbonate, diethyl carbonate, ethyl methyl carbonate ordipropyl carbonate; and a mixed solvent of a cyclic carbonate, anon-cyclic carbonate and an aliphatic carboxylic acid ester such asmethyl formate, methyl acetate, methyl propionate or ethyl propionate,and the like.

[0046] As the lithium salt, LiClO₄, LiAsF₆, LiPF₆, LiBF₄, LiCF₃SO₃,LiN(CF₃SO₂)(C₂F₅SO₂), LiN(CF₃SO₂)₂, LiN(C₂F₅SO₂)₂, LiN(CF₃SO₂)(C₄F₉SO₂)or the like can be used. These may be used alone or in combination oftwo or more.

[0047] The positive electrode can be produced, for example, by preparinga positive electrode mixture comprising a positive electrode activematerial, binder and conductive material, and applying the same to ancurrent collector. The negative electrode can be produced, for example,by preparing a negative electrode mixture comprising a negativeelectrode material and binder, and applying the same to a currentcollector.

[0048] As the binder for the negative electrode, either one ofthermoplastic resin and thermosetting resin may be employed. Thesematerials may be used alone or in combination. Particularly preferredare: styrene butadiene rubber, polyvinylidene fluoride, anethylene-acrylic acid copolymer or an Na ion-cross-linked copolymerthereof, an ethylene-methacrylic acid copolymer or an Naion-cross-linked copolymer thereof, an ethylene-methyl acrylatecopolymer or an Na ion-cross-linked copolymer thereof, anethylene-methyl methacrylate copolymer or an Na ion-cross-linkedcopolymer thereof, and the like.

[0049] As the conductive agent for the negative electrode, artificialgraphite, acetylene black, carbon fiber or the like can be used. Theamount of the conductive agent is preferably 1 to 50 wt% with respect tothe negative electrode material, and more preferably 1 to 30 wt%. In thecase where the negative electrode material itself has electronicconductivity, the conductive agent may not be added.

[0050] The current collector for the negative electrode is preferablymade of copper or copper alloy. The surface of these materials may beoxidized for use. Additionally, it is preferable to form asperities onthe surface of the current collector. As the current collector, metallicfoil, film, sheet or net; punched metal; metal lath; porous material,foamed material, woven fabric or nonwoven fabric each havingconductivity, or the like may be used. The thickness of the currentcollector is 1 to 500 μm, although not specifically limited thereto.

[0051] As the conductive agent for the positive electrode, artificialgraphite, acetylene black or the like can be employed. The amount of theconductive agent is preferably 1 to 50 wt% with respect to the positiveelectrode active material, and more preferably 1 to 30 wt%, although notspecifically limited thereto. In the case of carbon or graphite, it ismost preferably 2 to 15 wt%.

[0052] As the binder for the positive electrode, either one ofthermoplastic resin and thermosetting resin can be used. These materialscan be used alone or in combination. Particularly preferred materialsare polyvinylidene fluoride and polytetrafluoroethylene.

[0053] The current collector for the positive electrode is preferablymade of aluminum or aluminum alloy. The surface of these materials maybe oxidized for use. Additionally, it is preferable to form asperitieson the surface of the current collector. As the current collector,metallic foil, film, sheet or net; punched metal; metal lath; porousmaterial, foamed material, woven fabric or nonwoven fabric each havingconductivity, or the like may be used. The thickness of the currentcollector is 1 to 500 μm, although not specifically limited thereto.

[0054] In the electrode mixture, a filler or other various additives canbe contained, in addition to the conductive agent and the binder. Theamount of the filler is preferably 0 to 30 wt% in the electrode mixture,although not specifically limited thereto.

[0055] As the separator used in the present invention, microporous thinfilm or nonwoven fabric each having high ionic permeability, apredetermined mechanical strength and insulating property is preferablyemployed. It is preferable that the separator has the function toincrease the resistance, by closing its pores at a certain temperature.It is desirable that the pore size of the separator is in a range wherethe active material, binder and conductive agent, each of which isoccasionally released from the electrode, do not pass therethrough; forexample, 0.01 to 1 μm. Generally, the thickness of the separator is 10to 300 μm. The porosity of the separator is generally 30 to 80%.

[0056] The positive electrode mixture and negative electrode mixture maycontain a gel comprising a polymer material and a liquid non-aqueouselectrolyte retained therein. It is also possible to configure a batterycomprising a positive electrode, a negative electrode and a porousseparator comprising such gel integrated therewith. As theabove-described polymer material, for example, a copolymer of vinylidenefluoride and hexafluoropropylene is preferable.

[0057] Next, the present invention is concretely described withreference to examples. Herein, descriptions are made for squarebatteries; however, the shape of the battery is not limited thereto. Thepresent invention is also applicable to batteries of coin-type,button-type, sheet-type, laminated-type, cylindrical-type and flat-type.The present invention is also applicable to large batteries for use inelectric vehicles and the like.

EXAMPLE 1

[0058] (i) Fabrication of Positive Electrode

[0059] A positive electrode active material having the compositionformula (Li_(0.95)Al_(0.05))(Co_(0.9)Al_(0.1))O₂ was synthesized in thefollowing manner.

[0060] An aqueous solution containing cobalt sulfate at a concentrationof 0.9 mol/l and aluminium sulfate at a concentration of 0.1 mol/l wasprepared. While adding dropwise an aqueous solution of sodium hydroxidesuch that the pH of the above solution was 10 to 13, the respectivematerials were continuously supplied to a reaction vessel thereby tosynthesize a precursor comprising a hydroxide (Co_(0.9)Al_(0.1)) (OH)₂.

[0061] The obtained precursor, lithium carbonate and aluminium hydroxidewere mixed such that the molar ratio of the total amount of cobalt andaluminium in the precursor, the amount of lithium in lithium carbonateand the amount of aluminium in aluminium hydroxide was 1:0.95:0.05.After temporarily baked at 600° C., the mixture was pulverized andsubsequently baked again at 900° C., followed by pulverization andclassification, thereby producing a positive electrode active material.It should be noted that the baking was performed in air for 10 hourseach time.

[0062] 100 parts by weight of the obtained positive electrode activematerial, 3 parts by weight of acetylene black as a conductive agent and7 parts by weight of polytetrafluoroethylene as a binder were mixed,followed by further adding thereto 100 parts by weight of an aqueoussolution of carboxymethyl cellulose having a concentration of 1 wt%, andthe whole was mixed and stirred to produce a pasty positive electrodemixture.

[0063] This positive electrode mixture was applied onto both sides of acurrent collector of aluminium foil having a thickness of 20 μm, whichwas dried, then rolled and cut into a predetermined size to produce apositive electrode.

[0064] (ii) Fabrication of Negative Electrode

[0065] 100 parts by weight of flake graphite, pulverized and classifiedto have a mean particle diameter of about 20 μm, was mixed with 3 partsby weight of styrenebutadiene rubber as a binder, followed by furtheradding thereto 100 parts by weight of an aqueous solution ofcarboxymethyl cellulose having a concentration of 1 wt%, and the wholewas mixed and stirred to produce a pasty negative electrode mixture.

[0066] This negative electrode mixture was applied onto both sides of acurrent collector of copper foil having a thickness of 15 μm, which wasdried, then rolled and cut into a predetermined size to produce anegative electrode.

[0067] (iii) Fabrication of Battery

[0068]FIG. 1 shows a partially cutaway oblique view of a squarenon-aqueous electrolyte secondary battery (width 34 mm, height 50 mm)fabricated in this example. As shown in the figure, the above-describedpositive electrode and negative electrode were spirally wound, with aseparator disposed therebetween, to form an electrode plate assembly 1.One end of a positive electrode lead 2 made of aluminium and one end ofa negative electrode lead 3 made of nickel were welded to the positiveelectrode and negative electrode, respectively. An insulating ring madeof polyethylene resin was mounted on an upper side of the electrodeplate assembly 1, and the whole was placed in a battery case 4 made ofaluminium. It should be noted that the insulating ring is not shown inthe figure. The other end of the positive electrode lead 2 wasspot-welded to a sealing plate 5 made of aluminum, and the other end ofthe negative electrode lead 3 was spot-welded to a negative electrodeterminal 6 made of nickel located at the center of the sealing plate 5.The opening end of the battery case 4 was laser-welded with the sealingplate 5, and a predetermined amount of a non-aqueous electrolyte wasinjected from an inlet into the battery. Finally, the inlet was closedwith a sealing stopper 7 made of aluminium, followed by laser-welding,thereby completing a battery.

[0069] Herein, a microporous polyethylene film having a thickness of 25μm was used as the separator. Further, a mixed solvent of ethylenecarbonate and ethyl methyl carbonate at a volume ratio of 1:3 with LiPF₆dissolved therein at a concentration of 1.0 mol/l, was used as thenon-aqueous electrolyte.

[0070] The battery thus fabricated was named Battery 1A of the presentinvention.

EXAMPLE 2

[0071] A precursor comprising a hydroxide (Co_(0.9)Cu_(0.1))(OH)₂ wassynthesized in the same manner as in Example 1 except for the use ofcopper sulfate in place of aluminium sulfate.

[0072] Further, the precursor was used to produce a positive electrodeactive material (Li_(0.95)Cu_(0.05))(Co_(0.9)Cu_(0.1))O₂ in the samemanner as in Example 1 except for the use of copper carbonate in placeof aluminium hydroxide, and this was used to fabricate Battery 2Asimilar to Battery 1A.

EXAMPLE 3

[0073] A precursor comprising a hydroxide (Co_(0.9)Zn_(0.1))(OH)₂ wassynthesized in the same manner as in Example 1 except for the use ofzinc sulfate in place of aluminium sulfate.

[0074] Further, the precursor was used to produce a positive electrodeactive material (Li_(0.95)Zn_(0.5)) (Co_(0.9)Zn_(0.1))O₂ in the samemanner as in Example 1 except for the use of zinc oxide in place ofaluminium hydroxide, and this was used to fabricate Battery 3A similarto Battery 1A.

EXAMPLE 4

[0075] A precursor comprising a hydroxide (Co_(0.9)Mg_(0.1))(OH)₂ wassynthesized in the same manner as in Example 1 except for the use ofmagnesium sulfate in place of aluminium sulfate.

[0076] Further, the precursor was used to produce a positive electrodeactive material (Li_(0.95)Mg_(0.05)) (Co_(0.9)Mg_(0.1))O₂ in the samemanner as in Example 1 except for the use of basic magnesium carbonatein place of aluminium hydroxide, and this was used to fabricate Battery4A similar to Battery 1A.

EXAMPLE 5

[0077] A precursor comprising a hydroxide (Co_(0.9)Ca_(0.1))(OH)₂ wassynthesized in the same manner as in Example 1 except for the use ofcalcium sulfate in place of aluminium sulfate.

[0078] Further, the precursor was used to produce a positive electrodeactive material (Li_(0.95)Ca_(0.05))(Co_(0.9)Ca_(0.1))O₂ in the samemanner as in Example 1 except for the use of calcium carbonate in placeof aluminium hydroxide, and this was used to fabricate Battery 5Asimilar to Battery 1A.

EXAMPLE 6

[0079] A precursor comprising a hydroxide (Co_(0.9)Ba_(0.1))(OH)₂ wassynthesized in the same manner as in Example 1 except for the use ofbarium sulfate in place of aluminium sulfate.

[0080] Further, the precursor was used to produce a positive electrodeactive material (Li_(0.95)Ba_(0.05))(Co_(0.9)Ba_(0.1))O₂ in the samemanner as in Example 1 except for the use of barium carbonate in placeof aluminium hydroxide, and this was used to fabricate Battery 6Asimilar to Battery 1A.

EXAMPLE 7

[0081] A precursor comprising a hydroxide (Co_(0.9)Sr_(0.1))(OH)₂ wassynthesized in the same manner as in Example 1 except for the use ofstrontium sulfate in place of aluminium sulfate.

[0082] Further, the precursor was used to produce a positive electrodeactive material (Li_(0.95)Sr_(0.05))(Co_(0.9)Sr_(0.1))O₂ in the samemanner as in Example 1 except for the use of strontium carbonate inplace of aluminium hydroxide, and this was used to fabricate Battery 7Asimilar to Battery 1A.

EXAMPLE 8

[0083] An aqueous solution containing cobalt sulfate at a concentrationof 0.99 mol/l and aluminium sulfate at a concentration of 0.01 mol/l wasprepared. While adding dropwise an aqueous solution of sodium hydroxidesuch that the pH of the above solution was 10 to 13, the respectivematerials were continuously supplied to a reaction vessel thereby tosynthesize a precursor comprising a hydroxide (Co_(0.99)Al_(0.01))(OH)₂.

[0084] Except that the obtained precursor, lithium carbonate andaluminium hydroxide were mixed such that the molar ratio of the totalamount of cobalt and aluminium in the precursor, the amount of lithiumin lithium carbonate and the amount of aluminium in aluminium hydroxidewas 1:0.99:0.01, a positive electrode active material(Li_(0.99)Al_(0.01)) (Co_(0.99)Al_(0.01))O₂ was produced in the samemanner as in Example 1, and this was used to fabricate Battery 8Asimilar to Battery 1A.

EXAMPLE 9

[0085] A precursor comprising a hydroxide (Co_(0.99)Cu_(0.01))(OH)₂ wassynthesized in the same manner as in Example 8 except for the use ofcopper sulfate in place of aluminium sulfate.

[0086] Further, the precursor was used to produce a positive electrodeactive material (Li_(0.99)Cu_(0.01))(Co_(0.99)Cu_(0.01))O₂ in the samemanner as in Example 8 except for the use of copper carbonate in placeof aluminium hydroxide, and this was used to fabricate Battery 9Asimilar to Battery 1A.

EXAMPLE 10

[0087] A precursor comprising a hydroxide (Co_(0.99)Zn_(0.01))(OH)₂ wassynthesized in the same manner as in Example 8 except for the use ofzinc sulfate in place of aluminium sulfate.

[0088] Further, the precursor was used to produce a positive electrodeactive material (Li_(0.99)Zn_(0.01))(Co_(0.99)Zn_(0.01))O₂ in the samemanner as in Example 8 except for the use of zinc oxide in place ofaluminium hydroxide, and this was used to fabricate Battery 10A similarto Battery 1A.

EXAMPLE 11

[0089] A precursor comprising a hydroxide (Co_(0.99)Mg_(0.01))(OH)₂ wassynthesized in the same manner as in Example 8 except for the use ofmagnesium sulfate in place of aluminium sulfate.

[0090] Further, the precursor was used to produce a positive electrodeactive material (Li_(0.99)Mg_(0.01))(Co_(0.99)Mg_(0.01))O₂ in the samemanner as in Example 8 except for the use of basic magnesium carbonatein place of aluminium hydroxide, and this was used to fabricate Battery11A similar to Battery 1A.

EXAMPLE 12

[0091] A precursor comprising a hydroxide (Co_(0.99)Ca_(0.01))(OH)₂ wassynthesized in the same manner as in Example 8 except for the use ofcalcium sulfate in place of aluminium sulfate.

[0092] Further, the precursor was used to produce a positive electrodeactive material (Li_(0.99)Ca_(0.01))(Co_(0.99)Ca_(0.01))O₂ in the samemanner as in Example 8 except for the use of calcium carbonate in placeof aluminium hydroxide, and this was used to fabricate Battery 12Asimilar to Battery 1A.

EXAMPLE 13

[0093] A precursor comprising a hydroxide (Co_(0.99)Ba_(0.01))(OH)₂ wassynthesized in the same manner as in Example 8 except for the use ofbarium sulfate in place of aluminium sulfate.

[0094] Further, the precursor was used to produce a positive electrodeactive material (Li_(0.99)Ba_(0.01))(Co_(0.99)Ba_(0.01))O₂ in the samemanner as in Example 8 except for the use of barium carbonate in placeof aluminium hydroxide, and this was used to fabricate Battery 13Asimilar to Battery 1A.

EXAMPLE 14

[0095] A precursor comprising a hydroxide (Co_(0.099)Sr_(0.01))(OH)₂ wassynthesized in the same manner as in Example 8 except for the use ofstrontium sulfate in place of aluminium sulfate.

[0096] Further, the precursor was used to produce a positive electrodeactive material (Li_(0.99)Sr_(0.01))(Co_(0.99)Sr_(0.01))O₂ in the samemanner as in Example 8 except for the use of strontium carbonate inplace of aluminium hydroxide, and this was used to fabricate Battery 14Asimilar to Battery 1A.

EXAMPLE 15

[0097] An aqueous solution containing cobalt sulfate at a concentrationof 0.95 mol/l and aluminium sulfate at a concentration of 0.05 mol/l wasprepared. While adding dropwise an aqueous solution of sodium hydroxidesuch that the pH of the above aqueous solution was 10 to 13, therespective materials were continuously supplied to a reaction vesselthereby to synthesize a precursor comprising a hydroxide(Co_(0.95)Al_(0.05))(OH)₂.

[0098] Except that the obtained precursor, lithium carbonate andaluminium hydroxide were mixed such that the molar ratio of the totalamount of cobalt and aluminium in the precursor, the amount of lithiumin lithium carbonate and the amount of aluminium in aluminium hydroxidewas 1:0.97:0.03, a positive electrode active material(Li_(0.97)Al_(0.03))(Co_(0.95)Al_(0.05))O₂ was produced in the samemanner as in Example 1, and this was used to fabricate Battery 15Asimilar to Battery 1A.

EXAMPLE 16

[0099] A precursor comprising a hydroxide (Co_(0.95)Cu_(0.05))(OH)₂ wassynthesized in the same manner as in Example 15 except for the use ofcopper sulfate in place of aluminium sulfate.

[0100] Further, the precursor was used to produce a positive electrodeactive material (Li_(0.97)Cu_(0.03))(Co_(0.95)Cu_(0.05))O₂ in the samemanner as in Example 15 except for the use of copper carbonate in placeof aluminium hydroxide, and this was used to fabricate Battery 16Asimilar to Battery 1A.

EXAMPLE 17

[0101] A precursor comprising a hydroxide (Co_(0.95)Zn_(0.05))(OH)₂ wassynthesized in the same manner as in Example 15 except for the use ofzinc sulfate in place of aluminium sulfate.

[0102] Further, the precursor was used to produce a positive electrodeactive material (Li_(0.97)Zn_(0.03))(Co_(0.95)Zn_(0.05))O₂ in the samemanner as in Example 15 except for the use of zinc oxide in place ofaluminium hydroxide, and this was used to fabricate Battery 17A similarto Battery 1A.

EXAMPLE 18

[0103] A precursor comprising a hydroxide (Co_(0.95)Mg_(0.05))(OH)₂ wassynthesized in the same manner as in Example 15 except for the use ofmagnesium sulfate in place of aluminium sulfate.

[0104] Further, the precursor was used to produce a positive electrodeactive material (Li_(0.97)Mg_(0.03))(Co_(0.95)Mg_(0.05))O₂ in the samemanner as in Example 15 except for the use of basic magnesium carbonatein place of aluminium hydroxide, and this was used to fabricate Battery18A similar to Battery 1A.

EXAMPLE 19

[0105] A precursor comprising a hydroxide (Co_(0.95)Ca_(0.05))(OH)₂ wassynthesized in the same manner as in Example 15 except for the use ofcalcium sulfate in place of aluminium sulfate.

[0106] Further, the precursor was used to produce a positive electrodeactive material (Li_(0.97)Ca_(0.03))(Co_(0.95)Ca_(0.05))O₂ in the samemanner as in Example 15 except for the use of calcium carbonate in placeof aluminium hydroxide, and this was used to fabricate Battery 19Asimilar to Battery 1A.

Example 20

[0107] A precursor comprising a hydroxide (Co_(0.95)Ba_(0.05))(OH)₂ wassynthesized in the same manner as in Example 15 except for the use ofbarium sulfate in place of aluminium sulfate.

[0108] Further, the precursor was used to produce a positive electrodeactive material (Li_(0.97)Ba_(0.03))(Co_(0.95)Ba_(0.05))O₂ in the samemanner as in Example 15 except for the use of barium carbonate in placeof aluminium hydroxide, and this was used to fabricate Battery 20Asimilar to Battery 1A.

Example 21

[0109] A precursor comprising a hydroxide (Co_(0.95)Sr_(0.05))(OH)₂ wassynthesized in the same manner as in Example 15 except for the use ofstrontium sulfate in place of aluminium sulfate.

[0110] Further, the precursor was used to produce a positive electrodeactive material (Li_(0.97)Sr_(0.03))(Co_(0.95)Sr_(0.05))O₂ in the samemanner as in Example 15 except for the use of strontium carbonate inplace of aluminium hydroxide, and this was used to fabricate Battery 21Asimilar to Battery 1A.

COMPARATIVE EXAMPLE 1

[0111] An aqueous solution containing cobalt sulfate at a concentrationof 0.85 mol/l and aluminium sulfate at a concentration of 0.15 mol/l wasprepared. While adding dropwise an aqueous solution of sodium hydroxidesuch that the pH of the above aqueous solution was 10 to 13, therespective materials were continuously supplied to a reaction vesselthereby to synthesize a precursor comprising a hydroxide(Co_(0.85)Al_(0.15))(OH)₂.

[0112] The obtained precursor and lithium carbonate were mixed such thatthe molar ratio of the total amount of cobalt and aluminium in theprecursor and the amount of lithium in lithium carbonate was 1:1. Aftertemporarily baked at 600° C., the mixture was pulverized andsubsequently baked again at 900° C., followed by pulverization andclassification, thereby producing a positive electrode active materialLi(Co_(0.85)Al_(0.15))O₂. It should be noted that the baking wasperformed in air for 10 hours each time.

[0113] The resultant positive electrode active material was used tofabricate Battery 1B similar to Battery 1A.

COMPARATIVE EXAMPLE 2

[0114] A precursor comprising a hydroxide (Co_(0.85)Cu_(0.15))(OH)₂ wassynthesized in the same manner as in Comparative Example 1 except forthe use of copper sulfate in place of aluminium sulfate.

[0115] Further, a positive electrode active materialLi(Co_(0.85)Cu_(0.15))O₂ was produced in the same manner as inComparative Example 1 except for the use of the precursor, and this wasused to fabricate Battery 2B similar to Battery 1A.

COMPARATIVE EXAMPLE 3

[0116] A precursor comprising a hydroxide (Co_(0.85)Zn_(0.15))(OH)₂ wassynthesized in the same manner as in Comparative Example 1 except forthe use of zinc sulfate in place of aluminium sulfate.

[0117] Further, a positive electrode active materialLi(Co_(0.85)Zn_(0.15))O₂ was produced in the same manner as inComparative Example 1 except for the use of the precursor, and this wasused to fabricate Battery 3B similar to Battery 1A.

COMPARATIVE EXAMPLE 4

[0118] A precursor comprising a hydroxide (Co_(0.85)Mg_(0.15))(OH)₂ wassynthesized in the same manner as in Comparative Example 1 except forthe use of magnesium sulfate in place of aluminium sulfate.

[0119] Further, a positive electrode active materialLi(Co_(0.85)Mg_(0.15))O₂ was produced in the same manner as inComparative Example 1 except for the use of the precursor, and this wasused to fabricate Battery 4B similar to Battery 1A.

COMPARATIVE EXAMPLE 5

[0120] A precursor comprising a hydroxide (Co_(0.85)Ca_(0.15))(OH)₂ wassynthesized in the same manner as in Comparative Example 1 except forthe use of calcium sulfate in place of aluminium sulfate.

[0121] Further, a positive electrode active materialLi(Co_(0.85)Ca_(0.15))O₂ was produced in the same manner as inComparative Example 1 except for the use of the precursor, and this wasused to fabricate Battery 5B similar to Battery 1A.

COMPARATIVE EXAMPLE 6

[0122] A precursor comprising a hydroxide (CO_(0.85)Ba_(0.15))(OH)₂ wassynthesized in the same manner as in Comparative Example 1 except forthe use of barium sulfate in place of aluminium sulfate.

[0123] Further, a positive electrode active materialLi(Co_(0.85)Ba_(0.15))O₂ was produced in the same manner as inComparative Example 1 except for the use of the precursor, and this wasused to fabricate Battery 6B similar to Battery 1A.

COMPARATIVE EXAMPLE 7

[0124] A precursor comprising a hydroxide (CO_(0.85)Sr_(0.15))(OH)₂ wassynthesized in the same manner as in

Comparative Example 1 except for the use of strontium sulfate in placeof aluminium sulfate.

[0125] Further, a positive electrode active materialLi(Co_(0.85)Sr_(0.15))O₂ was produced in the same manner as inComparative Example 1 except for the use of the precursor, and this wasused to fabricate Battery 7B similar to Battery 1A.

COMPARATIVE EXAMPLE 8

[0126] An aqueous solution of cobalt sulfate was prepared. While addingdropwise an aqueous solution of sodium hydroxide such that the pH of theabove aqueous solution was 10 to 13, the materials were continuouslysupplied to a reaction vessel thereby to synthesize a precursorcomprising cobalt hydroxide.

[0127] The obtained precursor, lithium carbonate and aluminium sulfatewere mixed such that the molar ratio of the amount of cobalt in theprecursor, the amount of lithium in lithium carbonate and the amount ofaluminium in aluminium sulfate was 1:0.85:0.15. After temporarily bakedat 600° C., the mixture was pulverized and subsequently baked again at900° C., followed by pulverization and classification, thereby producinga positive electrode active material (Li_(0.85)Al_(0.15))CoO₂. It shouldbe noted that the baking was performed in air for 10 hours each time.

[0128] The resultant positive electrode active material was used tofabricate Battery 8B similar to Battery 1A.

COMPARATIVE EXAMPLE 9

[0129] A positive electrode active material (Li_(0.85)Cu_(0.15))CoO₂ wasproduced in the same manner as in Comparative Example 8 except for theuse of copper sulfate in place of aluminium hydroxide, and this was usedto fabricate Battery 9B similar to Battery 1A.

COMPARATIVE EXAMPLE 10

[0130] A positive electrode active material (Li_(0.85)Zn_(0.15))CoO₂ wasproduced in the same manner as in Comparative Example 8 except for theuse of zinc sulfate in place of aluminium hydroxide, and this was usedto fabricate Battery 10B similar to Battery 1A.

COMPARATIVE EXAMPLE 11

[0131] A positive electrode active material (Li_(0.85)Mg_(0.15))CoO₂ wasproduced in the same manner as in Comparative Example 8 except for theuse of magnesium sulfate in place of aluminium hydroxide, and this wasused to fabricate Battery 11B similar to Battery 1A.

COMPARATIVE EXAMPLE 12

[0132] A positive electrode active material (Li_(0.85)Ca_(0.15))CoO₂ wasproduced in the same manner as in Comparative Example 8 except for theuse of calcium sulfate in place of aluminium hydroxide, and this wasused to fabricate Battery 12B similar to Battery 1A.

COMPARATIVE EXAMPLE 13

[0133] A positive electrode active material (Li_(0.85)Ba_(0.15))CoO₂ wasproduced in the same manner as in Comparative Example 8 except for theuse of barium sulfate in place of aluminium hydroxide, and this was usedto fabricate Battery 13B similar to Battery 1A.

COMPARATIVE EXAMPLE 14

[0134] A positive electrode active material (Li_(0.85)Sr_(0.15))CoO₂ wasproduced in the same manner as in Comparative Example 8 except for theuse of strontium sulfate in place of aluminium hydroxide, and this wasused to fabricate Battery 14B similar to Battery 1A.

COMPARATIVE EXAMPLE 15

[0135] Lithium carbonate and cobalt hydroxide were mixed such that thenumbers of moles of lithium and cobalt were equal to synthesize apositive electrode active material LiCoO₂ under the same conditions asin Example 1, and this was used to fabricate Battery 15B similar toBattery 1A.

[0136] [Evaluation of Batteries]

[0137] Batteries 1A to 21A of the examples of the present invention andBatteries 1B to 15B of the comparative examples were subjected tocharge/discharge cycles at an ambient temperature of 20° C. In charging,a constant voltage charge was performed for two hours with a maximumcurrent of 600 mA and an end of charge voltage of 4.2 V. Discharge wasperformed at a constant current of 600 mA and an end of dischargevoltage of 3.0 V.

[0138] The ratio of the battery capacity after 300 cycles to the batterycapacity at 1st cycle was calculated as a capacity retention rate inpercentage. In addition, the increase in battery thickness occurredafter charge/discharge cycles was measured. The results are shown inTable 1. TABLE 1 Capacity Thickness Element Composition of positiveretention increase Battery added electrode active material rate [%] [mm] 1A Al (Li_(0.95)Al_(0.05))(Co_(0.9)Al_(0.1))O₂ 92 0.010  2A Cu(Li_(0.95)Cu_(0.05))(Co_(0.9)Cu_(0.1))O₂ 92 0.008  3A Zn(Li_(0.95)Zn_(0.05))(Co_(0.9)Zn_(0.1))O₂ 90 0.013  4A Mg(Li_(0.95)Mg_(0.05))(Co_(0.9)Mg_(0.1))O₂ 91 0.006  5A Ca(Li_(0.95)Ca_(0.05))(Co_(0.9)Ca_(0.1))O₂ 92 0.017  6A Ba(Li_(0.95)Ba_(0.05))(Co_(0.9)Ba_(0.1))O₂ 89 0.017  7A Sr(Li_(0.95)Sr_(0.05))(Co_(0.9)Sr_(0.1))O₂ 92 0.015  8A Al(Li_(0.99)Al_(0.01))(Co_(0.99)Al_(0.01))O₂ 90 0.062  9A Cu(Li_(0.99)Cu_(0.01))(Co_(0.99)Cu_(0.01))O₂ 89 0.058 10A Zn(Li_(0.99)Zn_(0.01))(Co_(0.99)Zn_(0.01))O₂ 88 0.062 11A Mg(Li_(0.99)Mg_(0.01)) 89 0.055 (Co_(0.99)Mg_(0.01))O₂ 12A Ca(Li_(0.99)Ca_(0.01))(Co_(0.99)Ca_(0.01))O₂ 88 0.069 13A Ba(Li_(0.99)Ba_(0.01))(Co_(0.99)Ba_(0.01))O₂ 88 0.070 14A Sr(Li_(0.99)Sr_(0.01))(Co_(0.99)Sr_(0.01))O₂ 90 0.060 15A Al(Li_(0.97)Al_(0.03))(Co_(0.95)Al_(0.05))O₂ 91 0.030 16A Cu(Li_(0.97)Cu_(0.03))(Co_(0.95)Cu_(0.05))O₂ 91 0.029 17A Zn(Li_(0.97)Zn_(0.03))(Co_(0.95)Zn_(0.05))O₂ 89 0.032 18A Mg(Li_(0.97)Mg_(0.03)) 90 0.025 (Co_(0.95)Mg_(0.05))O₂ 19A Ca(Li_(0.97)Ca_(0.03))(Co_(0.95)Ca_(0.05))O₂ 90 0.038 20A Ba(Li_(0.97)Ba_(0.03))(Co_(0.95)Ba_(0.05))O₂ 89 0.038 21A Sr(Li_(0.97)Sr_(0.03))(Co_(0.95)Sr_(0.05))O₂ 91 0.037  1B AlLi(Co_(0.85)Al_(0.15))O₂ 82 0.22  2B Cu Li(Co_(0.85)Cu_(0.15))O₂ 80 0.20 3B Zn Li(Co_(0.85)Zn_(0.15))O₂ 78 0.25  4B Mg Li(Co_(0.85)Mg_(0.15))O₂80 0.19  5B Ca Li(Co_(0.85)Ca_(0.15))O₂ 81 0.27  6B BaLi(Co_(0.85)Ba_(0.15))O₂ 79 0.26  7B Sr Li(Co_(0.85)Sr_(0.15))O₂ 82 0.22 8B Al Li(Co_(0.85)Al_(0.15))CoO₂ 84 0.19  9B CuLi(Co_(0.85)Cu_(0.15))CoO₂ 81 0.17 10B Zn Li(Co_(0.85)Zn_(0.15))CoO₂ 800.21 11B Mg Li(Co_(0.85)Mg_(0.15))CoO₂ 82 0.23 12B CaLi(Co_(0.85)Ca_(0.15))CoO₂ 82 0.24 13B Ba Li(Co_(0.85)Ba_(0.15))CoO₂ 810.25 14B Sr Li(Co_(0.85)Sr_(0.15))CoO₂ 84 0.18 15B None Li(CoO₂ 74 0.32

[0139] Comparison between Batteries 1B to 7B and Battery 15B as well ascomparison between Batteries 8B to 14B and Battery 15 show that partlyreplacing either one of lithium and cobalt in LiCoO₂ with a differentelement improved, to some extent, the capacity retention rate andsuppressed an increase in battery thickness.

[0140] Comparison between Batteries 1A to 21A and Batteries 1B to 14Bdemonstrates that partly replacing both of lithium and cobalt in LiCoO₂with a different element was more effective than partly replacing one ofthem with the same.

[0141] Comparison between Batteries 8A to 14A and Batteries 1B to 14Bdemonstrates that partly replacing both of lithium and cobalt with asmall amount of a different element was more effective than partlyreplacing one of them with a large amount of the same.

[0142] Moreover, as compared with the batteries of the comparativeexamples, the batteries employing the positive electrode activematerials of the present invention exhibited a greater improvement intemperature rising test and reliability test at high temperatures, eachusing the batteries in a charged state and those subjected tocharge/discharge cycles, so that they had higher reliability underabnormal conditions.

Industrial Applicability

[0143] According to the present invention, it is possible to provide apositive electrode active material capable of yielding a non-aqueouselectrolyte secondary battery with improved thermal stability of whichincrease in thickness and decrease in discharge capacity due to repeatedcharge/discharge cycles are suppressed.

1. A positive electrode active material for a non-aqueous electrolytesecondary battery, comprising a composite oxide represented by thegeneral formula (1):(Li_(1−x)M_(x))_(a)(CO_(1−y)M_(y))_(b)O_(c)  (1)where lithium and cobaltare partly replaced with element M in a crystal structure of LiCoO_(c),wherein element M is at least one selected from the group consisting ofAl, Cu, Zn, Mg, Ca, Ba and Sr, and
 0. 02≦x+y≦0.15,0.90≦a/b≦1.10, and1.8≦c≦2.2 are satisfied.
 2. The positive electrode active material for anon-aqueous electrolyte secondary battery in accordance with claim 1,wherein said composite oxide has an α—NaFeO₂ type layer structurebelonging to space group R3-m.
 3. The positive electrode active materialfor a non-aqueous electrolyte secondary battery in accordance with claim1, wherein said composite oxide has a mean particle diameter of 5 to 20μm.
 4. The positive electrode active material for a non-aqueouselectrolyte secondary battery in accordance with claim 1, wherein saidcomposite oxide has a specific surface area of 0.3 to 1.2 m²/g.
 5. Anon-aqueous electrolyte secondary battery comprising: a positiveelectrode comprising the positive electrode active material inaccordance with any one of claims 1 to 4; a negative electrodecomprising metallic lithium or a material capable of absorbing anddesorbing lithium; and a non-aqueous electrolyte.